Inverse feedback amplifier



April 1941. R. A. BIERWIRTH INVERSE FEEDBACK AMPLIFIER Filed July 29. L939 Patented Apr. 29, 1941 INVERSE FEEDBACK AMPLIFIER Rudolph A. Bierwirth, Haddon Heights, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application July 29, 1939, Serial No. 287,305

8 Claims.

The present invention relates to inverse feedback amplifiers of the type wherein a portion of the output voltage is fed back to the input circuit through a separate feedback circuit from a feedback winding included in the amplifier outformer of a balanced audio frequency amplifier is provided in addition to an output secondary or load circuit winding, with a separate feedback winding connected in series between the cathode of an input amplifier stage and ground thereby to provide a series inverse feedback connection for the first amplifier stage which is substantially without load and through which substantially distortionless inverse feedback is provided.

It is an object of the present invention to provide an inverse feedback amplifier of the character referred to, having an improved feedback circuit and an improved coupling means for minimizing phase shift in the feedback loop, whereby the feedback action maybe increased without causing oscillation in the amplifier circuit.

It is also an object of the present invention to provide an improved feedback circuit and feedback coupling means for an audio frequency amplifier which may carry an appreciable load substantially without phase shift in the feedback voltage or current and without setting up oscillations in the amplifier with a relatively high degree of feedback.

It is a further object of the present invention to provide an improved output circuit and transformer coupling arrangement for an inverse feedback amplifier which provides relatively low leakage reactance between the inverse feedback winding and the primary winding whether the feedback circuit is loaded as in shunt inverse fecdback circuits or substantially unloaded as in series inverse feedback circuits.

It is a still further object of the present invention to provide an improved arrangement of coupling windings and connections for an output transformer adapted for inverse feedback operation with a balanced or push-pull amplifier.

Other objects and advantages of the invention will be understood from the following description when considered in connection with the accompanying drawing and the scope of the invention will be pointed out in the appended claims.

In the drawing, Figure 1 is a schematic circuit diagram of an inverse feedback amplifier provided with an output and feedback circuit embodying the invention;

Figure 2 is an end elevation of a transformer adapted for use in the circuit of Figure 1 with the windings spaced to indicate their relation to each other and to the core; and

Figure 3 is a side elevation of the transformer of Fig. 2, partly in section, taken on the line 3 -3 of Fig. 2.

Referring to Fig. 1, in the audio frequency amplifier shown, an amplifier tube 5 in the first stage is suitably coupled to signal input terminals 6 through a coupling capacitor 1. A grid resistor 8 is connected between a signal input or control grid 9 of the amplifier tube 5 and ground or chassis, indicated at H].

The amplifier 5 may be of any suitable type of control grid having a cathode II, and an output anode [2. The latter is coupled through suitable coupling means [3 and a phase inverter stage of any known construction indicated at M to a pair of push-pull or balanced output tubes l5l6 which supply the amplified audio frequency energy from the amplifier to a secondary or load winding IT in an output transformer I8 through a balanced anode output circuit Ill-20 in which is located the balanced primary winding 2! for the tubes.

Primary winding 2! is provided with a center tap 22 which is connected to a positive anode potential supply means represented by the lead 23, the negative B supply being the chassis or ground. The output circuit l92fi is provided with a shunt capacitor and resistance load circuit 24-25, having a center tap 26 to ground for correcting the leakage reactance of the transformer secondary I1 when connected to a load.

A tertiary winding 3ll as a source of feedback voltage without phase shift is connected between ground 3| and the cathode ll of the input amplifier stage 5 through a feedback connection 32 and a feedback voltage bleeder or potentiometer resistor comprising a resistor section 33 and a second resistor section 34 in the cathode lead and connected to ground 35 to complete the feedback and cathode circuits. The feedback voltage developed in the winding 39 is applied between the cathode H and grid 9 of the input stageacross the resistor section 34 in opposition to the incom ing signal potential.

The resistor 33, together with the resistor 34, is designed to provide a relatively light load in shunt with the feedback winding 30 and the resistor sections 33 and 34 are so proportioned that a maximum feedback from. the output circuit may be obtained without oscillation. Compared to the inverse feedback amplifiers heretofore known providing a feedback of substantially 5 to db. the circuit shown may provide an inverse feedback of 28 db. without oscillation.

This is for the reason that a low leakage tertiary winding is provided, the low leakage being obtained by close coupling the winding 30 to the primary or output circuit l 92ll. The close coupling is obtained by the use of an additional primary winding which may be termed the inverse feedback primary comprising two winding sections 36 and 31 also on the common core 33 with the primary, load secondary, and tertiary windings and arranged on either side of the tertiary winding as shown, to provide a split or divided primary winding for close coupling with the tertiary 30. Conversely, the tertiary winding may be divided and arranged in the manner described for the primary to provide a similar close coupling.

The inverse feedback primary winding sections 36 and 31 are serially connected across the output circuit l9 -2ll of the amplifier through connection leads 43 and M and are further provided with a common center connection with the main primary 2i and the supply lead 23, through leads 42 and 43. This places the winding 33 in parallel with one-half of the main primary winding 2! While the remaining winding 37 is similarly con nccted with the other half of the winding 2| and provides close coupling between the primary cir cuit. By this arrangement and connection of the windings, the leakage loss is reduced to a minimum. The relatively light loading on the tertiary circuit provided by the series bleeder element 33 materially contributes to this result.

The resistor 34 in the inverse feedback circuit being common to the anode circuit of the first stage amplifier is of such resistance value that a predetermined negative bias is established therein for the control grids 9. Hence, the resistor 34 may be of relatively low value with respect to the series resistor 33 in the feedback circuit. The latter resistor may be considered as an isolating resistor between the feedback winding 30 and the cathode circuit on the input or first stage amplifier 5. In practice, the windings l1, 2|, 30, 36 and 31 are preferably arranged on the transformer core as shown in Figs. 2 and 3 and in which the same reference numerals are applied to corresponding parts as in Fig. 1.

Referring to Figs. 2 and 3 along with Fig. 1, the core 38 is provided with an inner winding which is one section 31, for example, of the inverse feedback primary. The next outer winding is the feedback secondary 30, outside of which is placed the second section 36 of the inverse feedback primary. The outermost winding is the main primary 2| and between the primary 2| and the one section 36 of the inverse feedback primary is placed the main or load secondary l1.

It has been found that this arrangement of windings provides effective coupling between the load circuit, secondary l1, and the main primary 2|, while effectively coupling the tertiary or feedback winding 30 with the primary circuit substantially without leakage reactance and corresponding phase shift even when the feedback circuit is loaded to a predetermined degree.

As hereinbefore pointed out, the inverse feedback system shown and described is particularly adapted for amplifiers requiring a high degree of feedback without oscillation. The phase shift is reduced to a minimum by the coil arrangement and connections shown.

I claim a my invention:

1. In an inverse feedback audio frequency amplifier, the combination with an output amplifier stage, of an output transformer therefor, a feedback Winding in said transformer, an inverse feedback circuit including said winding serially therein, means for coupling said winding with minimum leakage reactance to said output stage comprising a primary winding connected with said stage and having two spaced sections between which said feedback winding is interposed, and a common core member for said windings.

2. An audio frequency amplifier having an output transformer provided with a feedback windins, an inverse feedback circuit connected there with, a signal amplifier stage having an output circuit and means for coupling said output circuit with said feedback winding with a minimum leakage reactance comprising a primary winding connected with said output circuit, and a common core member for said windings, one of said windings being divided to provide two spaced sections with the other of said windings located therebetween on a common portion of said core member.

3. In an inverse feedback audio frequency amplifier, an output transformer having two primary windings, an output circuit to which said windings are connected, 2. load secondary winding coupled to one of said primary windings, a feedback winding coupled to the other of said primary windings, said last-named primary winding and feedback winding being tightly coupled and one of said windings being divided to provide two spaced sections with the other winding interposed therebetween, and an inverse feedback circuit in which said feedback winding is serially connected. whereby said feedback winding is subject to a minimum leakage reactance and load thereby to reduce phase distortion to a minimum in said feedback circuit.

4. In an inverse feedback audio frequency amplifier, an output transformer having two primary windings, an output circuit to which said windings are connected, a load secondary winding coupled to one of said primary windings, a feedback winding coupled to the other of said primary windings, said last-named primary winding and feedback winding being tightly coupled and one of said windings being divided to provide two spaced sections with the other Winding interposed therebetwecn, and an inverse feedback circuit in which said feedback winding is serially connected, means providing a common core element for said windings including a central portion about which said windings are concentrically arranged, the first-named primary being the outermost winding with respect to said portion of the core, and one section of the divided winding being the innermost winding with respect to the said portion of the core.

5. In an inverse feedback amplifier, compris- .ing an input amplifier stage and an output amplifier stage, the combination of an output coupling transformer for said last named stage having a main primary winding, a secondary load winding and a tertiary winding, means providing inverse feedback circuit connections between said first amplifier stage and said tertiary winding, and means additional to the main primary winding providing coupling for the primary winding with said tertiary winding and a low leakage reactance for said windings.

6. In an inverse feedback amplifier, the com bination of a balanced output circuit for said amplifier, an output transformer having a balanced primary winding connected with said balanced output circuit, a secondary providing load circuit coupling with said primary Winding, an inverse feedback primary winding having two substantially equal sections each connected in parallel to one half of the balanced primary winding and arranged. in spaced relation to each other in said transformer, and a tertiary feedback winding located between said sections and closely coupled therewith to provide a relatively low leakage reactance, and an inverse feedback circuit including said tertiary winding as a series element thereof.

'7. In an inverse feedback audio frequency amplifier, the combination with an inverse feedback circuit, of a feedback winding included serially in said circuit, an output transformer for said amplifier including said winding and having a primary winding and a main load secondary winding, an output circuit for said amplifier connected with said primary winding, and means for coupling said feedback winding with said output circuit comprising an additional primary winding connected across said output circuit and comprising two sections arranged on opposite sides of said feedback winding and tightly coupled therewith to provide relatively low leakage reactance, and means in said feedback circuit for reducing the load on said feedback Winding to a predetermined low value.

8. In an inverse feedback audio frequency amplifier, the combination of an amplifier stage having an amplifier tube provided with a cathode resistor of predetermined value for biasing said tube, an output for said stage, an output transformer for said amplifier having a feedback winding, a Winding connected with said output circuit and coupled with said feedback winding, one of said last-named coupling and feedback windings being divided to provide two sections between which the other winding is inserted, thereby to provide relatively low leakage reactance and tight coupling, means providing a series inverse feedback circuit connection through said inverse feedback winding and said cathode resistor, and a series impedance device in circuit between said inverse feedback winding and said cathode resistor having a relatively high resistance with respect to said cathode resistor, thereby to prevent loading of said feed- 

